A Radio Frequency Identification (RFID) reader is a transmitter/receiver that reads the contents of RFID tags in the vicinity. Also called an “RFID interrogator” the maximum distance between the reader's antenna and the tag vary, depending on application.
Various diversity techniques have been deployed to improve the quality and reliability of reader antennas. For example, spatial diversity has been employed that use multiple antennas, usually with same characteristics, that are physically separated from one another.
Pattern diversity is another technique that has been employed. Pattern diversity typically consists of two or more co-located antennas with different radiation patterns. This type of diversity makes use of directive antennas that are usually physically separated by some distance.
Another technique is polarity diversity which combines pairs of antennas with orthogonal polarizations (i.e., horizontal, vertical, slanted). With polarity diversity, the same information signal is transmitted and received simultaneously or alternately on orthogonally polarized waves.
One limitation of these techniques is that they do not effectively deal with environmental or antenna null zones. In a null zone, an RFID tag cannot be interrogated by the reader as there is no electromagnetic energy within the null zone to excite the coil of the RFID tag. In addition, many of these techniques require the use of multiple antennas. Multiple antennas, however, can present additional problems. For example, multiple antennas in close proximity can couple to one another, thereby creating additional nulls. This is especially problematic in the near field since the coupling between the antennas can be particularly strong.
Accordingly, it would be advantageous to develop an RFID reader that could alleviate the effect of nulls and at the same time provide the benefits of antenna diversity in communicating with tags.